Multifocal contact lens with aspheric surface

ABSTRACT

A contact lens is disclosed having a front surface and a back surface. The lens contains a continuously varying aspheric surface on one or more of these surfaces.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an improved lens design. Morespecifically, the present invention relates to an improved multifocallens using one or more aspheric surfaces for vision correction.

[0002] A spherical lens has a front and back surface with each surfacehaving a constant radius of curvature. The focal power of the sphericallens is also constant. As you move along the lens in a radial directionfrom the center point to the periphery, the optical power of thespherical lens does not change except for smaller order effects due tooptical aberration.

[0003] An aspheric lens on the other hand has a non-constant radius ofcurvature on one or both of its front and back surfaces. The focal powerof the aspheric lens changes as you move along the radius of the lens.This feature is the basis for a multifocal vision correcting lens.

[0004] The degree to which an aspheric lens departs from a sphericallens is measured by the eccentricity parameter e. If e=0, the lens has aspherical surface. If e=1 the lens has a parabolic surface; if e>1 thelens has a hyperbolic surface, and if e<1 the lens has an ellipticalsurface.

[0005] One use of the aspheric lens, particularly a contact lens, is tocorrect presbyopia (a vision condition associated with age). Over timethe presbyopic patient loses visual accommodation (i.e., the ability ofthe eye to change optical power in order to adjust focus for differentviewing distances) such that objects at near or intermediate viewingdistances are not seen clearly without the aid of a near power lens. Theaspheric lens compensates for presbyopia by providing a range of opticalpower including that required for far, near, and intermediate viewingdistances. Generally, by increasing the eccentricity e, the range ofoptical power provided by the aspheric lens increases such that thevalue of e may in principle be adjusted for early or advancedpresbyopia. However, there appears to be a maximum eccentricity valuewhich is useful. With current designs with e values below approximately0.8, additional near power of up to approximately +1.50 D is possible.This is suitable for early to moderate presbyopia. For moderate toadvanced presbyopia +1.50 to +2.50 D (or more) of additional near powerare required. However, if the eccentricity e is increased aboveapproximately 0.8 to provide this increased level of additional nearpower, it is found that the quality of distance vision becomes socompromised as to be unacceptable to most patients.

[0006] In U.S. Pat. No. 4,704,016, a multifocal contact lens isdisclosed. The major viewing area of the lens is divided into amultiplicity of near and distant vision viewing zones. The wearer isable to simultaneously look through at least two zones of differentpower. One way of creating the zones is to form a series of concentricrings using a lathe. The annular area of the lens is cut alternately fordistant and for near vision correction. The eccentricity of the surfaceis varied in dependence on the radius but there is no dependence on theequatorial angle φ. Another technique disclosed in the patent is toincorporate segments of material having a different refractive indexfrom that of the body of the lens. The eccentricity of these lenses isalso independent of the equatorial angle φ. These lenses do not solvethe problem of channeling too much light into the near vision portion ofthe lens.

[0007] U.S. Pat. No. 4,898,461 discloses a lens similar to U.S. Pat. No.4,704,016. Like the foregoing disclosure the lens has a plurality ofalternating focal power zones. Here, the focal power varies continuouslyin the radial direction within each zone and in the transition areabetween each zone. The eccentricity of these lenses is independent ofthe equatorial angle φ. These lenses also do not solve the problem ofchanneling too much light into the near vision portion of the lens.

[0008] Another contact lens design has been proposed for achieving nearand distant vision correction known as the translating design.Translating designs attempt to exploit the fact that when a wearer looksdown to read, a contact lens rides up on the wearer's cornea.Translating designs thus attempt to place an optical zone with thedistance power over the pupil of the eye when the patient is lookingstraight ahead and an optical zone with the near power over the pupilwhen the patient is looking down to read. However, sufficient andreliable translation has not been achieved to make the lens satisfactoryin most applications. Also, the comfort of translating designs is oftenunacceptable to many patients.

[0009] There is a need for an improved multifocal lens which eliminatessome or all of these problems found in the prior art lens designs.

SUMMARY OF THE INVENTION

[0010] The present invention provides, according to a first aspect, acontact lens having a front surface, a back surface and an apex. Thelens defines a series of adjacent points at a fixed distance from theapex. The series of adjacent points on the lens having a continuouslyvarying power, the series of adjacent points extending across an arc ofat least 120°.

[0011] According to another aspect of the invention, the contact lensincludes a front surface and a back surface. One of the front surfaceand the back surface is an aspheric surface wherein the eccentricityvaries continuously as a function of the angle φ.

[0012] According to yet another aspect of the invention, a bottomportion of the lens has an eccentricity that varies continuously as afunction of the angle φ and a top portion of the lens has asubstantially constant eccentricity as a function of the angle φ.

[0013] According to a further aspect of the invention, the lens includestwo side portions that have an eccentricity that varies continuously asa function of the angle φ and top and bottom portions that have asubstantially constant eccentricity as a function of the angle φ.

[0014] The lens of the present invention has several advantages overprior lenses including an enhanced visual acuity at near and distancepowers. In addition, the present invention overcomes the add powerproblem of previous aspheric lenses while retaining the advantages of anaspheric lens, i.e., to provide an intermediate vision capability.

[0015] These and other aspects and features of the invention will befurther understood when considered in conjunction with the followingdetailed description of the embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 illustrates a perspective view of an embodiment of thepresent invention and the cylindrical coordinate system (ρ,φ,z) used todescribe the embodiment;

[0017]FIG. 2 illustrates an embodiment of the present inventionpositioned on an eye along with an angular coordinate system used todescribe the embodiment.

[0018]FIG. 3 is a diagram illustrating a ray tracing method used in thecalculation of the Add power as a function of half chord diameter;

[0019]FIG. 4 is a graph showing the Add power as a function of the halfchord diameter for e_(min) for a first embodiment (BC represents basecurve; BVP represents back vertex power; e represents eccentricity; Drepresents Diopter);

[0020]FIG. 5 is a graph showing the Add power as a function of the halfchord diameter for e_(max) for a first embodiment (BC represents basecurve; BVP represents back vertex power; e represents eccentricity; Drepresents Diopter);

[0021]FIG. 6 is a graph showing the angular dependence of eccentricityfor first, second and third embodiments of the present invention (e−1represents the eccentricity in the first embodiment; e−2 represents theeccentricity in the second embodiment; e−3 represents the eccentricityin the third embodiment; φ represents the equatorial angle); and

[0022]FIG. 7 is a contour map of the aspheric surface according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS AND THE PREFERRED EMBODIMENTS

[0023] The invention has presently found particular application as alens for vision correction. However, the invention is considered to havefar ranging applications and potential adaptations and should not be solimited.

[0024] A preferred embodiment of the invention is shown in FIG. 1 as acontact lens 10. The lens 10 has an optically transparent body 12 with afront surface 14 and a back surface 16. The back surface 16 is basicallyconcave shaped and is adapted to fit the curvature of the wearer's eyein a conventional manner. The front surface 14 includes an asphericsurface having an eccentricy e that varies continuously as a function ofthe equatorial angle φ across a portion of the lens.

[0025] To describe this eccentricity, reference is made to thecylindrical coordinate system depicted in FIG. 1. In FIG. 1 the z-axisis also the optical axis of the lens, and the orientation of the lens issuch that it is concave in the direction of the positive z-axis. Thisparticular orientation of the lens with respect to the z-axis in FIG. 1is one which is commonly assumed for the programming of computercontrolled lathes used in lens manufacture. Although other coordinatesystems could be used, the cylindrical coordinate system chosen providesthe advantage that standard forms for the conic sections may be used tospecify the aspheric profile of the surface in terms of an angledependent eccentricity variable e.

[0026] In the cylindrical coordinate system of FIG. 1 the position of anarbitrary point P is specified in terms of the parameters ρ,φ and z. Theparameter ρ is the radial distance of the point from the z-axis. Theparameter φ is the angle between the xz-plane and the plane thatcontains both the z-axis and the point P. The parameter z is thedistance along the z-axis. These parameters may assume the followingranges of values:

−∞≦Z≦∞

0≦ρ≦∞

0°≦φ≦360°

[0027] Accordingly, the parameters ρ, φ and z can represent any point Pon, or in, the lens 10.

[0028] For convenience, the origin O of the cylindrical coordinatesystem coincides with the apex of the front surface of the lens 10, andthe z-axis coincides with the optical axis of the lens which can beoffset from the geometrical axis. Then if we let:

e=e(φ)=eccentricity as a function of the angle φ

g=g(φ)=1−e²

j=j(φ)=1/g=1/(1−e²)

r₀=apical radius of the aspheric surface=optical radius for the apicalpower

[0029] then for any given value of φ the relationship between ρ and zmay be expressed as follows:

[0030] (a) For the case where the front surface is aspheric and thecenter of the lens 10 has a focal power adapted for distance vision:

ρ²=2r₀z−jz² (where 0<e<1 and j>1)

with ρ=ρ(φ,z)=(2r₀z−jz²)^(½)

and z=(ρ²r₀)/[1+(1−jρ²/r₀ ²)^(½)]and

[0031] (b) For the case where the back surface is aspheric and thecenter of the lens 10 has focal power adapted for distance vision:

ρ²=2r₀z−gz².

with ρ=ρ(φ,z)=(2r₀z−gz²)^(½)

and z=(ρ²/r₀)/[1−gρ²/r₀ ²)^({fraction (1/1)})]

[0032] As shown, the φ dependence of ρ(φ,z) in the above equations isdetermined entirely by the variables j or g which in turn are functionsonly of e(φ).

[0033] The present invention encompasses embodiments where the center ofthe lens has a power adapted for near vision or for distance vision.However, for illustration purposes, only the center distance embodimentswill be discussed. The present invention also encompasses embodimentsthat have aspheric surfaces on either the front or back surface, or onboth surfaces. Only the front surface configuration is illustrated inthe embodiments discussed below.

[0034] Given the relationship between ρ and z, the following descriptionis directed to three different embodiments, each using a differentfunction e(φ). As those of ordinarily skill in the art will recognize,other relationships between e and φ come within the scope of the presentinvention. For example, factors such as the desired additional nearpower, the centration and movement of the lens on the eye, and patientcharacteristics such as pupil size may lead to other preferredrelationships between e(φ).

[0035] For each of the embodiments, it is convenient and conventional tofurther describe an angular orientation with respect to the lens interms of a clock face. FIG. 2 shows the lens 10 mounted on an eye 18between the upper eyelid 20 and the lower eyelid 22. Looking at thefront surface 14 of the lens 10 the 12 o'clock position is up, the 6o'clock position is down, and the 3 o'clock and 9 o'clock positions areto the right and left respectively. In order to make the angle φ of FIG.1 consistent with the ophthalmic system for specifying angularorientation with respect to the eye, the positive x-axis is placed atthe 3 o'clock position and the positive y-axis is placed at the 12o'clock position in FIG. 2. The positive z-axis is therefore pointingout of the patient's eye (i.e., out of the page) in FIG. 2. In contrastto FIG. 1 the lens on the eye is now concave in the direction of thenegative z-axis. Fortunately, this difference between the manufacturingsystem of FIG. 1 and the ophthalmic system of FIG. 2 is not a source forconfusion, since only a change in the sign of z is required to switchfrom one system to the other.

[0036] In the first embodiment, the near correction will be concentratedin the 4 to 8 o'clock region with an eccentricity that is greater thanapproximately 0.8. The 10 to 2 o'clock region will have distancecorrection with an eccentricity that is less than approximately 0.8.And, over the entire lens 10, the eccentricity could be on averageapproximately 0.8. However, other eccentricity values can be implementedon a lens construed in accordance with the present invention.

[0037] The eccentricity changes continuously from its maximum valuee_(max) at 6 o'clock and its minimum value e_(min) at 12 o'clock in thefirst embodiment. The function of e(φ) is described as follows:

e(φ)=A−Bsin(φ)for φ=0° to 360°

[0038] where the constants A and B are defined by

A=(e_(max)+e_(min))/2 and B=(e_(max)−e_(min))/2

or e_(max =e()270°)=A+B and e_(min)=e(90°)=A−B

[0039] A second embodiment of the present invention has a configurationwhere the function e(φ) remains constant at its minimum value in the tophalf of the lens (from 9 o'clock to 12 o'clock to 3 o'clock) and thefunction e(φ) changes continuously (from 3 o'clock to 6 o'clock to 9o'clock) to a maximum value at 6 o'clock. The second embodiment mayoffer a slightly better distance vision but slightly worse near visionthan the first embodiment. The following equations define the functione(φ) for this embodiment:

e(φ)=A for φ=0° to 180°

e(φ)=A−Bsin(φ) for φ=180° to 360°

[0040] where the constants A and B are defined by

A=e_(min) and B=e_(max)−e_(min)

or e_(max)=e(270°)=A+B and e_(min)=e(90°)=A

[0041] The function e(φ) does not have to be sinusoidal to be cyclicalin φ. A third embodiment has a configuration where the function e(φ)remains constant at its minimum value in a top region (from 10 o'clockto 12 o'clock to 2 o'clock) and remains constant at its maximum value inan inferior region (from 4 o'clock to 8 o'clock). In the nasal andtemporal regions function e(φ) changes linearly between a maximum and aminimum level. The following equations describe the function e(φ) forthis embodiment:

e(φ)=e_(max)−(e_(max)−e_(min))(φ+30°)/60° for φ=0° to 30°

e(φ)=e_(min) for φ=30° to 150°

e(φ)=e_(min)+(e_(max)−e_(min))(φ−150°)/60° for φ=150° to 210°

e(φ)=e_(max) for φ=210° to 330°

e(φ)=e_(max)−(e_(max)−e_(min))(φ−330°)/60° for φ=330° to 360°

[0042] The three sample functions for e(φ) presented above are expressedin terms of the quantities, e_(max) and e_(min). These quantities arefunctions of the distance power, the desired near Add power, the basecurve, center thickness, and the refractive index of the lens material.To calculate a specific example for illustration purposes, the followingbaseline values are used:

[0043] Base Curve 8.800 mm

[0044] Center Thickness 0.130 mm

[0045] Refractive Index 1.412

[0046] Apical Back Vertex Power +1.00 D

[0047] Target Add for e_(min) +1.25 D at a 1.6 mm half chord diameter

[0048] Target Add for e_(max) +2.50 D at a 1.6 mm half chord diameter

[0049] Front Apical Radius 8.6539 mm

[0050] The 1.6 mm half chord diameter (the distance from the center axisto a point on the surface) corresponds to a 3.2 mm pupil diameter. Thefront apical radius value is what is needed to provide the chosen apicalback vertex power.

[0051] The Add power may be found by various methods such as by directmathematical computation, by graphical construction, by ray tracing, andthe like. For example, applying the baseline values above to theaspheric front surface distance center configuration, the Add power as afunction of half chord diameter may then be computed by tracing rays foran axial object at infinity as shown in FIG. 3. The eccentricity of thefront surface is then adjusted until the desired Add power is achieved.For example, e_(min) =0.7588 provides a +1.25 D Add power at a 1.6 mmhalf chord diameter. Similarly, e_(max) =0.8527 provides a +2.50 D Addpower at the same 1.6 mm half chord diameter. The Add power profilesfound by ray tracing for these values of e_(max) and e_(min) at otherhalf chord diameters are plotted in FIGS. 4 and 5.

[0052] The angular dependence of the functions e(φ) for the threeembodiments are illustrated in FIG. 6. In FIG. 6, all three functionsare calculated for the same values of e_(max) and e_(min), hence, allhave the Add power profiles shown in FIG. 4 for 12 o'clock and in FIG. 5for 6 o'clock. As stated previously, the appropriate function e(φ)depends on several factors and must be selected based on the particularapplication. Factors such as ease of manufacture, cost and overall lensperformance should be considered. For example, the second embodiment mayoffer slightly better distance vision but at the cost of slightly worsenear vision than the first embodiment. It may be desirable to simplifythe angle dependence further for the sake of manufacturing ease or cost.Thus, the result of the sinusoidal function in e(φ) might beapproximated by a sinusoidal function in ρ which is expressed in termsof ρ_(max) and ρ_(min) instead of e_(max) and e_(min).

[0053] The aspheric surface resulting from the first functionalrelationship, e(φ)=A−B sin (φ), is represented by a contour map in FIG.7. To obtain the contours in the figure, the cross-section of theaspheric surface is plotted in the XY plane for constant values of z.Note that the variation of ρ with φ is small and that all of thecontours in the figure may appear to be circles. However, they are notcircles. To better illustrate that the contours actually deviateslightly from perfect circles, a few sample values of ρ are also given.At z=0.100 mm it may be seen that ρ is 1.3066 mm at 12 o'clock and1.3016 mm at 6 o'clock. Thus, the difference between the maximum andminimum value of ρ is only 5 microns at this value of z. Near the apex,at z=0.010 mm, the difference between the maximum and minimum ρ is onlya tenth of a micron, while at z=0.700 mm the difference is about 100microns. This value of z corresponds to an aspheric optical zone ofroughly 6.6 mm.

[0054] Lenses employing the present invention may be made onconventional manufacturing equipment such as a lathe. If a lathingprocess is used, then the axis of the spindle is most conveniently theZ-axis of the cylindrical coordinate system of FIG. 1, and the positionof the cutting tool during lathing is given by ρ(φ,z). During eachrevolution of the spindle the cutting tool must alternately move closerto and farther from the spindle axis as it cycles through the minimumand maximum ρ values for the current value of z. As discussed above forthe surface in FIG. 7, the magnitude of the excursion of the cuttingtool over its range of values during each rotation cycle is only afraction of a micron near the apex of the surface. At larger z values,the magnitude of the excursion of the cutting tool increases tosomething on the order of 100 microns or more depending on the size ofthe aspheric optical zone.

[0055] Although specific values for refractive index, base curve, centerthickness, back vertex power, and Add power targets are given for thesample lens computation discussed above, the form of the equations andthe method of calculation is general and can be applied to other values.One method of manufacturing the present invention is to select anexisting sphere or toric lens series and then graft the desired asphericoptical zone onto the front surface. In this approach the maincomputational task for each member of the lens series will be tocalculate the required e_(max) and e_(min) values which provide thedesired Add powers.

[0056] It may be desirable to evaluate a range of values for e_(max) ande_(min) to investigate the relationship between the calculated power,the measured power, and the clinical power effect. In the abovediscussion, the calculated apical power is used as a designation for thedistance power. However, since the power changes continuously with theaspheric profile, it is possible that the clinical distance power effectis somewhat more plus (or less minus) than the apical power. Also, thedesignated Add power in the above discussion is based arbitrarily on thecalculated Add power at a 1.6 mm half chord diameter. The usefulattainable Add power is limited by the degree of asphere-induced imagedegradation that most wearers will accept.

[0057] The lens body 12 can be constructed from material to form a hard,gas permeable, or soft contact lens. While the size of lens may beadjusted to suit a wearer's eye size, the preferred outer diameter iswithin the range of approximately 8.0 mm to 15.5 mm. The asphericsurface can be on the front or back surfaces or both. The power can becenter distance or near. And, if needed a toric feature may be added toone of the surfaces. In the described preferred embodiments theasphericity is on the front surface, the toric surface, if required,would be on the base curve, and the center of the lens 10 is designedfor distance correction with the peripheral part of the optical zonedesigned for near correction.

[0058] To provide the necessary stabilization to orient the region ofmaximum Add power at the 6 o'clock position on the eye, any of a numberof methods to prevent lens rotation may be used. For example, aconventional prism ballast may be used to achieve rotational stability.

[0059] In manufacturing embodiments of the present invention, coordinatesystems are chosen by convention, and the system adopted for thisdiscussion is selected primarily because the equations for ρ(φ,z) canthen be written in a very concise form. Since the function e(φ) may beselected such that it is smooth and continuous over the whole surface,machining should not be difficult. Machining of the lenses can be donedirectly using a fast tool servo system. Suitable machines are providedby Moore and Rank-Pneumo. Lenses may also be formed by molding startingfrom masters which are manufactured according to the disclosedmathematical functions to drive the tooling. An alternate mathematicalformulation is to use spherical harmonics or other appropriate expansionthat provides a series expansion in terms of an amplitude moderated byan angular term. The equipment selected for the fabrication process mayrequire a different coordinate system, but once this is identified itshould be relatively straightforward to perform the necessarytransformations between the two systems.

[0060] The embodiments described above and shown herein are illustrativeand not restrictive. The scope of the invention is indicated by theclaims rather than by the foregoing description and attached drawings.The invention may be embodied in other specific forms without departingfrom the spirit of the invention. For example, linear and non-linearchanges in eccentricity come within the scope of the present invention.Changes that come within the scope of the claims are intended to beembraced herein.

We claim:
 1. A contact lens comprising: a front surface, a back surfaceand an apex, the lens defining a series of adjacent points at a fixeddistance from the apex, the series of adjacent points on the lens havinga continuously varying power, the series of adjacent points extendingacross an arc of at least 120°.
 2. The contact lens of claim 1 whereinthe front surface is an aspheric surface.
 3. The contact lens of claim 1wherein the back surface is an aspheric surface.
 4. The contact lens ofclaim 1 wherein the series of adjacent points extend across a bottomportion of the lens.
 5. The contact lens of claim 4 wherein a secondseries of adjacent points at a fixed distance from the apex extendacross a top portion of the lens, the second series of adjacent pointsdefining a substantially constant power.
 6. The contact lens of claim 5wherein the second series of adjacent points extend across an arc of180°.
 7. The contact lens of claim 6 wherein the top portion has aminimum power and the bottom portion includes a maximum power.
 8. Thecontact lens of claim 1 wherein the series of adjacent points are on aside portion of the lens.
 9. The contact lens of claim 8 wherein thelens includes a plurality of predefined regions, each region having asubstantially constant power along an arc of points equal distance froma center of the lens.
 10. The contact lens of claim 9 wherein the topportion has a minimum power and the bottom portion includes a maximumpower.
 11. The contact lens of claim 1 wherein the lens has ageometrical center and an optical center, the optical center beingoffset from the geometrical center.
 12. A contact lens comprising: afront surface and a back surface, one of the front surface and the backsurface being an aspheric surface wherein an eccentricity of theaspheric surface varies continuously as a function of the angle φ. 13.The contact lens of claim 12 wherein the eccentricity varies accordingto the following equation: e(φ)=A−B sin (φ) for φ=0° to 360° where theconstants A and B are defined by A=(e_(max)+e_(min))/2 andB=(e_(max)−e_(min))/2 e_(max)=e(270°)=A+B and e_(min)=e(90°)=A−B
 14. Thecontact lens of claim 13 wherein the aspheric surface is the frontsurface.
 15. The contact lens of claim 14 wherein the aspheric surfaceis the back surface.
 16. A contact lens comprising: a top portion and abottom portion, the top portion having a constant eccentricity as afunction of the angle φ, the bottom portion having an eccentricity thatvaries continuously as a function of the angle φ.
 17. The contact lensof claim 16 wherein the top portion has an eccentricity to provide adistance correction power and the bottom portion has an eccentricity toprovide, in part, a near correction power.
 18. The contact lens of claim17 wherein the near correction power has a maximum correction powerwhere the angle φ is in the range 225°-315°.
 19. The contact lens ofclaim 18 wherein the near correction power has a maximum correctionpower where the angle φ is 270°.
 20. The contact lens of claim 19wherein the eccentricity of the bottom portion varies by the function:e(φ)=A−B sin (φ)for φ=180° to 360° where the constants A and B aredefined by A=e_(min) and B=e_(max)−e_(min) e_(max)=e(270°)=A+B ande_(min)=e(90°)=A
 21. The contact lens of claim 20 wherein the topportion and the bottom portion are on a back surface.
 22. The contactlens of claim 21 wherein the top portion and bottom portion are on afront surface.
 23. The contact lens of claim 22 wherein the lensincludes a ballast.
 24. A contact lens comprising: a top portion and abottom portion, and two opposite side portions, the top portion having afirst eccentricity along a selected arc, the bottom portion having asecond eccentricity different from the first eccentricity along theselected arc and the side portions having an eccentricity that thatvaries continuously as a function of the angle φ along the selected arc.25. The contact lens of claim 24 wherein the top portion has aneccentricity that provides a distance correction power and the bottomportion has an eccentricity that provides a near correction power. 26.The contact lens of claim 25 wherein the first side portion is found atφ=150° to 210° and the second side portion is found at φ=330° to 360°and 0° to 30°.
 27. The contact lens of claim 26 wherein the eccentricityof the side portions varies according to the following equations:e(φ)=e_(max)−(e_(max)−e_(min))(φ+30°)/60° for φ=0° to 30°e(φ)=e_(min)+(e_(max)−e_(min))(φ−150°)/60° for φ=150° to 210°e(φ)=e_(max)−(e_(max)−e_(min))(φ−330°)/60° for φ=330° to 360°
 28. Thecontact lens of claim 27 wherein the top portion and the bottom portionare on a front surface.
 29. The contact lens of claim 28 wherein thelens includes a prism ballast.
 30. A contact lens comprising: a frontsurface and a back surface, one of the front surface and the backsurface being an aspheric surface wherein an eccentricity of theaspheric surface varies continuously as a function of the angle φ,wherein a near correction power is located between 30°-150° and adistance correction power is located between 210°-330°.